The present invention relates to a transmission apparatus for transmitting information by means of an Orthogonal Frequency Division Multiplex (OFDM) system and more particularly to the technique for improving the accuracy of detection of synchronization of a demodulator in a receiver.
In the OFDM system, a multiplicity of carriers having phases orthogonal to one another and having a narrow frequency band width are arranged in a given frequency band for communication. When a signal modulated by the OFDM system and transmitted is to be demodulated by a receiver, it is necessary to pick up synchronization information from the OFDM signal which, before attaining synchronization, looks like noise and then demodulate the signal.
As measures for taking synchronization between a receiver and transmitter, an example is described in JP-A-7-030513, in which no signal sections (null sections) are previously inserted into the OFDM signal periodically in the transmitter so that the null sections are used as a reference to take synchronization.
Further, another example is described in xe2x80x9cA Study on Field Pickup Unit Using OFDM Modulation Schemexe2x80x9d by Shigeki et al, Institute of Television Engineers of Japan, Technical Report vol., 19, No. 38, pp. 7-12, August 1995, in which a group of synchronization symbols including a null section, a sweep signal (which varies from a lower limit frequency to an upper limit frequency of a transmission band during one symbol) and the like is inserted in the beginning of a frame which is a unit for data transmission processing and synchronization is taken using the synchronization symbol group.
The configuration of a demodulator used in the above example is shown in FIG. 2.
In this demodulator, an RF signal modulated by the OFDM system is converted into an IF frequency by a receiving unit RF 90 and then converted into a baseband by a receiving unit IF 91. The converted signal is digitized by an A/D converter 62 and is supplied to an OFDM demodulator 93. In synchronization detection, an output signal of the receiving unit RF 90 is subjected to a square-law detection and synchronization detection in a synchronization detector 92 and an output signal of the synchronization detector 92 is supplied to the OFDM demodulator 93.
JP-A-7-321762 discloses a technique in which sample values of the same synchronization symbol waveform (sweep signal) as that in a transmitter are stored in a receiver and correlation values of the sample values of the synchronization symbol waveform and sample values of a received signal are calculated in a correlation calculator, so that a clock frequency in the receiver is controlled to be coincident with a clock frequency of the received signal.
First, the prior art requires a high-frequency analog circuit for effecting the square-law detection and the synchronization detection with respect to the output signal of the receiving unit RF 90 of FIG. 2 which is an analog high-frequency signal and it is not easy to realize a stable synchronization detection circuit providing for mixing of noise. Second, when noise is mixed in the null section or multipath and fading occur in the section in which data is transmitted resulting in a reduced reception level even if the synchronization detection is digitally processed, there occurs the problem that the data transmission section is mistaken as the no signal section and finally the synchronization detection cannot be detected stably.
Waveforms of signals at portions of FIG. 2 are now described with reference to FIGS. 3A and 3B.
The output of the receiving unit RF 90 of FIG. 2 has a certain amplitude during transmission of data as shown by an OFDM received signal 71 of FIG. 3A and the null sections of N1, N2 and N3 are provided periodically.
Accordingly, the OFDM demodulator 93 detects the null sections at points N1 and N2 of the OFDM received signal 71 of FIG. 3A to produce synchronization detection signals 72 (S1 and S2) as shown in FIG. 3B. However, when a level is reduced due to influence of the multipath and the fading as shown by a section F1 of the OFDM received signal 71, a signal S3 is produced in error.
Further, as shown by the null section N3 of the OFDM received signal 71, when noises are mixed in this null section, the null section cannot be sometimes detected as shown by the synchronization detection signal 72.
Accordingly, it is an object of the present invention to provide an orthogonal frequency division multiplex data transmission apparatus capable of effecting detection of synchronization stably.
According to an aspect of the present invention, in order to achieve the above object, a transmitter of the data transmission apparatus using the orthogonal frequency division multiplex modulation system previously inserts a group of predetermined synchronization symbols into a transmission signal every fixed period in order to synchronize a receiver with the transmitter and the receiver demodulates a received transmission signal to a baseband OFDM signal. An absolute value of an A/D converted digital signal is taken and an obtained absolute signal is bandwidth-limited to a predetermined band width. Then, it is decided whether an amplitude of the signal is larger than or smaller than a predetermined value. A no signal period (hereinafter referred to as a null section) in the synchronization symbol group is detected on the basis of the decision result and a start point of another synchronization symbol subsequent to the null section is further detected. The time when both of the presence of the null section and the start point of the different synchronization symbol are detected is regarded as a synchronization timing and the operation timing of a demodulator for the receiver is set to the synchronization timing.
Further, the comparison and decision processing as to whether the amplitude of the signal converted into the absolute value and bandwidth-limited to the predetermined band width is larger than or smaller than the predetermined value is performed by using a comparator for making comparison as to whether the amplitude of the signal is larger than or smaller than the predetermined value and a counter for increasing or decreasing a count thereof in accordance with the comparison result and in the detection and decision processing of the null section, the point of time when the count is larger than or smaller than the predetermined value is detected as the start point of the null section.
Moreover, the detection of the start point of the another synchronization symbol subsequent to the null section is performed by using a majority decision type edge detector for deciding whether the start point is reached or not from signal states of N samples (N is an integer larger than or equal to 2) obtained from the comparison decision result.
In addition, the majority decision type edge detection means includes means for counting the number of times a signal of the another synchronization symbol which is produced subsequent to the null section exceeds a predetermined threshold value during a period of N samples and detecting a position of the start point of the another synchronization symbol on the basis of a count thereof and means for judging whether an arrangement of signal values of the another synchronization symbol during the period of N samples is a predetermined arrangement or not to detect the start point of the different synchronization symbol.
As has been described above, in the present invention, in order to detect the null section, the level of the digitized baseband OFDM signal is used to decide the null section to thereby digitize the detection processing of the null section, so that the operation is stabilized. Further, the level of the sweep signal subsequent to the null section is examined, so that the detection is hard to be influenced by noise even if noise is instantaneously mixed into the null section.
Further, even when multipath or fading occurs to thereby reduce the level of the received signal, the null section is confirmed by the above method and accordingly the probability of mistaking the null section can be reduced.
Furthermore, in the present invention, in order to synchronize the receiver with the transmitter, since the detection of the start point of the sweep signal after the null section of the received signal is not decided by only one sample and uses majority decision type edge detection in which decision is made on the basis of signal states of N samples, the start point of the sweep signal can be detected with higher accuracy even if noise is mixed into the null section of the received signal and the sweep signal subsequent to the null section.